The increase of the density and capacity of HDDs has been achieved by reduction of particles of a recording film that configures a recording medium and increasing a magnetic anisotropy field Hk thereof, and also by down-sizing of a recording element of a magnetic head that records digital information to the recording medium and improving the material of a recording element. In recent years, HDDs with perpendicular magnetic recording system, which is suitable for high density recording, have been mainly used, and further increase of the density and capacity in the future has been anticipated.
On the other hand, in accordance with the reduction of recording bits and magnetic particles, thermal fluctuation of recording magnetization is more likely to occur. Accordingly, it is preferred to increase the magnetic anisotropy field Hk and coercive force Hc of the recording film.
However, when the Hk of the recording layer is increased, the Hc is also increased to 5 k (Oe) or more. It is normally said that a recording field that is twice or more of coercive force Hc is required to perform saturation magnetic recording of digital data to a recording medium. Therefore, it has been required to intensify a recording field that is generated from a recording element of a magnetic head, and a saturation magnetic flux density Bs of a recording element film has been increased. However, as the Slater-Pauling curve has shown, the saturation magnetic flux density is peaked at approximately 2.4 (T). Therefore, recording and erasing of digital data becomes difficult, causing difficulty to proceed the increase of the density and capacity of the HDDs.
In order to resolve such a situation, a new recording system with microwave assist has been proposed (see for example a patent literature 1). In the new recording system, a spin torque oscillator (STO) is formed in a magnetic gap between a main magnetic pole and a trailing shield which are configuring a recording element of a magnetic head for perpendicular recording, the STO being configured with a multi-layered magnetic thin film, and a microwave magnetic field in an in-plane direction is generated by oscillation of the STO and is applied to the recording medium, exciting precession of a magnetization, so that magnetization reversal in a perpendicular direction is assisted. Specifically, a field generation layer (FGL) of the STO that is configured by the multi-layered magnetic thin film is made to oscillate at a high frequency, a leaking magnetic field that is generated from a surface thereof is applied to the recording medium, and thereby the microwave assist is performed. The above-described assist system is referred to as an internal-oscillation system (see for example the patent literature 1).
However, the STO has significant technical problems such as that:    (1) lamination of a multi-layered magnetic thin film is necessary and a process thereof is extremely complex;    (2) in order that the STO oscillates, a magnetic film with extremely high magnetic anisotropy is necessary;    (3) a current density of current that is applied to the STO is an unique control parameter of an oscillation frequency, and the control is difficult because the oscillation frequency drastically changes by slight change of the current density; and    (4) as the current density of current that is applied to the STO increases, a generated magnetic field (hereinafter, is also referred to as generation magnetic field) increases and the oscillation frequency also changes, and thereby it is difficult to arbitrarily control each parameter independently.
In contrast to the internal-oscillation system, an external-oscillation system head device has been proposed, in which a secondary coil is arranged in a magnetic recording gap between a main magnetic pole and a trailing shield that faces the main magnetic pole configuring a recording element of a magnetic head, high frequency current in a microwave band is driven from an outside oscillator to the secondary coil to generate a high frequency in-plane magnetic field in the magnetic recording gap; and the high frequency in-plane magnetic field is superimposed on a perpendicular recording field that is generated by the main magnetic pole to assist magnetization reversal. The above-described assist system is referred to as an external-oscillation system (see for example a patent literature 2).
The external-oscillation system enables to apply an in-plane alternate-current (AC) magnetic layer at a high frequency in the microwave band to a recording film of the recording medium in a superimposing manner, and due to the assist effect, the intensity of a perpendicular recording field that is required for magnetization reversal of a medium recording layer can be significantly reduced, the perpendicular recording field being generated by the main magnetic pole. Also, high-speed recording and erasing of data to a recording layer with intense coercive force Hc becomes possible. Further, because the alternate-current (AC) magnetic field in the in-plane direction in the microwave band is generated by driving high frequency current to the secondary coil from the outside oscillator, the external-oscillation system has characteristics that the internal-oscillation system of the STO does not have such as:    (1) a frequency control of the high frequency current in ppm order is possible;    (2) a control of the in-plane generation magnetic field becomes easy by controlling amplitude of the high frequency current;    (3) as a result, it becomes possible to control separately the frequency in the microwave band and the in-plane AC magnetic field to generate, and setting of a frequency that is tuned to the ferromagnetic resonance frequency fRm of the recording medium and an optimal design of the magnetic head which an optimal distribution with the perpendicular recording field is considered become possible; and    (4) mass production is easy because the magnetic head structure is simple.
As described above in the external oscillation system, recording current that corresponds to digital data is driven to a main coil arranged near the main magnetic pole to generate the perpendicular recording field to the recording medium from the main magnetic pole. Simultaneously with this operation, high frequency current in the microwave band is applied to the secondary coil arranged in the magnetic recording gap between the main magnetic pole and the trailing shield. Due to magnetic flux induction, soft magnetic films that form the main magnetic pole and the trailing shield that faces the main magnetic pole are magnetized at a high frequency, and thereby a high frequency magnetic field in the in-plane direction is generated in the magnetic recording gap. The high frequency magnetic field is leaked from the gap and applied to the recording layer, and is superimposed on the perpendicular recording field, thereby assisting the magnetization reversal of the recording layer.
In order to increase the assist effect, the driving frequency of the high frequency current that is applied to the secondary coil is preferably 10 G (Hz) or more. However, the frequency of the high frequency magnetic field generated in the magnetic recording gap is peaked at the ferromagnetic resonance frequency fR of the soft magnetic films that form the main magnetic pole and the trailing shield, so that only the current with the ferromagnetic resonance frequency or less can be driven. Herein, the fR is shown by a following expression (see for example a non-patent literature 5).fR=(γ/2π)·(Hk·4πMs)1/2  (1)
γ: gyromagnetic constant, Hk: anisotropy field, Ms: magnetization 4πMs Bs
Bs: saturation magnetization flux density
Also, the assist magnetic field in the in-plane direction also need to have a ferromagnetic field of 1 k (Oe) or more. According to the expression (1), it needs only to increase Hk and Bs of the soft magnetic films in order to increase the fR, A soft magnetic film that is practically used for the main magnetic pole of the recording element of the magnetic head is mainly FeCo series, and Bs is 2−2.4 (T), which is large, and Hk is 5-30 (Oe), which is small (see for example patent literatures 3 and 4). In order to drive current to the secondary coil at a high frequency around 10 G (Hz), which is effective for the magnetization reversal assist, or more and to apply the in-plane high frequency magnetic field of 1 k (Oe) or more to the recording film, soft magnetic films that have sufficiently high ferromagnetic resonance frequency fR and a recording element structure are required to accomplish.
Also in the external-oscillation system, a generation magnetic field is peaked when the main magnetic pole that forms the recording element is magnetically saturated even if excitation current is further increased, and also the linear response at a high frequency is less likely to accompany. As described above in the external-oscillation system, the perpendicular recording field is generated from the main magnetic pole due to the excitation of the main coil, and a tip of the main magnetic pole is magnetically saturated. In order to generate an intense assist magnetic field in the in-plane direction from the magnetic recording gap, it is required to drive current in a high frequency preventing the effect of the magnetic saturation of the main magnetic pole.
Also in the external-oscillation system, a formulation validates that application of a bias magnetic field to the soft magnetic films is effective to further increase the fR of the soft magnetic films (see for example the non-patent literature 5). A parameter h thereof is defined as follows.h=(HB+Hk)/Hk 
HB; bias magnetic field
When the HB is increased, the h is also increased, and thereby the fR is increased. However, a technology that realizes the application of the bias magnetic field with a practical recording element structure has not been obvious yet, so there has been necessity to make it obvious in detail.
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